This invention relates to equipment and methods used in measuring electrical resistance in a worksurface as a prediction for electrostatic discharge potential.
Static electricity is commonly defined as an electrical charge resulting from the imbalance of electrons on the surface of a material. Most people are quite familiar with the everyday effects of static electricityxe2x80x94it is the shock one receives when touching a doorknob after walking across a carpet. The technical name for the electrical shock just described is electrostatic discharge (ESD). ESD is technically described as the transfer of electrical charge between bodiesxe2x80x94for instance, a human hand and a doorknobxe2x80x94that are at different electrical potentials.
In most everyday situations, ESD can be a bother but rarely a problem. However, the problems resulting from ESD are magnified in industrial settings, where ESD is a major concern. Among the many problems that static discharge can cause are the unintentional ignition of flammable materials, damage to electronic components and systems, and the attraction of contaminants such as charged dust particles in clean room environments. Even centuries ago military forces were aware that ESD could cause the unintentional ignition of black powder. To alleviate this sometimes-catastrophic problem, ESD control measures were used as early as the 14th century to protect black powder stores.
Today, many industriesxe2x80x94from high tech manufacturing plants to businesses commonly thought of as xe2x80x9csmoke stackxe2x80x9d industriesxe2x80x94are concerned with ESD and its control, since controlling ESD can lead to a safer work environment and reduction or elimination of damage resulting from ESD. While nearly all industries are or should be concerned with controlling ESD, the concern is most acutely felt by businesses in the electronics industry. To give just a few examples of the damage that ESD can cause in the electronics industry, it can destroy or degrade semiconductor devices by changing operational characteristics, it can cause disruptions to the normal operation of an electronic systemxe2x80x94sometime leading to equipment failure, and in clean rooms it can cause charged particles to adhere tightly to the surface of a silicon wafer, resulting in distinct problems with wafer production and efficiency.
Given these problems and the economic damage that can result from them, control of ESD is a major concern and a complete industry has grown up around the field of ESD control. Typically, an ESD control program has many different facets, considering for example factors such as the nature of the particular business, product design that takes into account ESD risk factors, defining the level of ESD control needed in the particular setting, identifying areas where ESD control are critical, eliminating sources of ESD generation, dissipating and neutralizing ESD with appropriate techniques, and protecting products from the ESD that will inevitably occur. Although not all industries will need an ESD control program that addresses each of these factors, almost all industries that are concerned with controlling ESD have a need to test their facilities for ESD susceptibility and the effectiveness of control measures.
One critical component of an ESD control program is dissipating and neutralizing ESD with appropriate techniques. As expected, there are many different techniques. One very common protective measure is the use of ESD resistive coatings on worksurfaces such as floors and bench tops. Many highly effective ESD resistive coatings are available for use on floors and other worksurfaces and as a result, nearly all electronics manufacturing facilities use surface coatings of one kind or another. Such coatings are specifically designed to eliminate or minimize the triboelectric charge potential between objects, and have been found to be highly effective.
The effectiveness of the coatings, or the need for coatings in a particular area is determined through ESD testing. The present invention relates to equipment and methods used to test the ESD potential of worksurfaces. Manufacturers routinely test ESD potential as part of an overall ESD control program. Regular testing provides a measure of the need for ESD control measures in a specific work setting, or the ongoing effectiveness of an in-place ESD control device such as a floor of bench top coating.
The level of ESD protection provided by a worksurface is directly related to the time needed to discharge an object. It is known, for example, that the electrical resistance between two points can be correlated to the ESD potential, and electrical resistance values between two points are indicative of the effectiveness of the surface for resisting ESD. Accordingly, worksurface ESD testing focuses on measuring the electrical resistance of flooring materials, packaging materials, bench tops, and point to point and point to ground resistance. While in reality the time that it takes for an object to discharge is related to factors in addition to resistance, such as capacitance, contact resistance and discharge path, it has been found in practice that the resistance measurement provides an effective predictor of the potential for ESD, and through a measurement that is fairly easy to quantify.
Various standards have been developed for measuring the resistance of worksurfaces as a predictor of ESD potential. As an example, ANSI/ESD-S7.1-1994 is an accepted standard for resistive characterization of materials, floor materials. The Electrostatic Discharge Association has also published standards for worksurface resistance measurements at ESD S4.1-1997. These standards are used in many manufacturing facilities.
The equipment used to test worksurface resistivity according to the standards just mentioned is commercially available from various sources. For example, ESD Systems.com (www.esdsystems.com) offers several megohmmeters that are used in compliance with the standards noted above and other published ESD measurement standards. These megohmmeters comprise equipment in compliance with the standards noted and include two 5 lb electrodes, leads and an ohmmeter having appropriate voltage characteristics. In use, the electrodes and ohmmeter are carried to the location where testing is to take place. The electrodes are separated according to the standard specificationsxe2x80x94the separation depending upon the type of measurement being takenxe2x80x94and the leads are connected to the ohmmeter. The resistance between electrodes is then measured and the value is used as an indicator of ESD potential, and, for example, the effectiveness of the surface coating. The actual testing routines are spelled out in the appropriate standards.
While the equipment just described complies with the published standards, it can be cumbersome to make the actual measurements on an ongoing basis in the field. For example, the electrodes are relatively heavy and thus difficult to handle. Placing the electrodes on the worksurface at the appropriate separation can be a difficult task, since the separation should be measured with each testing measurement taken to assure compliance with the standard. Moreover, the electrodes are prone to being damaged through repeated use, and the testing method using individual electrodes is necessarily a xe2x80x9cbatchxe2x80x9d operation. That is, resistance measurements are taken at discrete points in a manufacturing facility. While an increase in the number of test locations helps provide a statistical prediction of ESD potential over a larger area of a manufacturing facility, the batch testing routine does not provide a ready method of testing the overall effectiveness of a worksurface other than at discrete test locations.
There is a real need therefore for improved equipment for measuring the resistance of worksurfaces.
The present invention provides an ESD testing apparatus and method that complies with published and industry standard worksurface testing techniques. In a preferred embodiment the present invention comprises a testing apparatus that is mounted on a frame having wheels that allow the unit to be easily moved from location to location for point measurements of worksurface resistance. Electrodes are carried on board the frame and are adjustable to be moved from a first, narrow or retracted position that allows the frame to be easily moved around a manufacturing facility to a second extended position in which the electrodes are spaced at industry standard spacing. The electrodes are capable of measuring resistance in either the retracted or extended positions. The electrodes are removable from mounts on the frame and may be used for point-to-point floor measurements, point to ground measurements, and bench to floor measurements, all according to industry standards.
The present invention further allows for continuous worksurface resistivity testing by measuring resistance values between the frame wheels, which in a selected mode function as the electrodes. The resistance of, for example, a floor surface may thus be measured on a continuous basis as the frame is moved over the floor. This allows for testing of ESD potential and evaluation of ESD risk for a large sample area rather than being limited to multiple single-point test locations, thereby allowing for greater statistical testing and evaluation of ESD risk potential of the surface. To facilitate testing while the device is being rolled over a surface, the wheels are electrically conductive, as for example when the wheels are coated with an electrically conductive material. An electrical connection is made between the conductive coating and an on-board ohmmeter. The wheels are movable between retracted and extended positions to change the spacing between the wheels during testing. In a retracted position the wheels easily fit into tight spaces. In an extended position, the wheels are separated from one another by industry standard spacing.
In one measurement mode, the inventive apparatus may be set to continuously measure the resistance between the wheels as the frame is rolled over the floor. When the frame is tipped into a standing position, a switch may be activated to switch the device into a testing mode using the electrodes rather than the wheels.
The apparatus includes linkage mechanisms for separately moving the electrodes and the wheels from their retracted positions to their extended positions.
The test equipment includes an industry standard ohmmeter in compliance with published standards and standard-compliant electrodes. The ohmmeter is mounted on the frame in a position such that the operator may easily read the test results as the frame is either moved from location to location, or while testing in-transit. The equipment is thus far more ergonomically designed than currently available test equipment. A toolbox is included on the frame to store tools and other supplies that are commonly used during the testing routines.